Here is what I would do. The PCs burst in and are confronted with the big bad demon of evilness. Make it abundantly clear that this thing is WAY beyond their abilities. It scoops some of the cultists and eats them nonchalantly, growing larger as it does. Everyone, not just the good cleric / paladin can feel this things evil radiate from its body. Fear creeps into your heart and you know that you screwed up and that this thing is your fault.
Now the PCs have some options.
1) Turn evil and serve the demon. Kind of lame but it could take the campaign to some interesting places.
2) Stand your ground and face the evil that you helped bring into this world. Your PCs will lose. You cannot save them from themselves. And you will just have to let the dice fall where they may. Their defeat could be the springboard to another campaign! Now, if your players are like mine then they may gear up for their doomed last stand and blindly ignore a means of escape. Don't be afraid to point that out. If the players are unanimous that they have to stay and fight, let them. And kill them. And then use their old PCs against their new ones.
3) Give the PCs an out. Let them drop their weapons and flee from the cultist lair; tails between their legs. Perhaps the demon is not pleased with the cultists that dared to summon it. How dare they disrupt its plans in the abyss! So as the demon eats these pesky mortals the PCs can quickly exit out the front door.
Now the campaign becomes "What do you do?" Do the PCs go to High Holy Temple of Light and seek out a way to defeat this dangerous foe? Do they flee into the wilderness and attempt to save refugees and rebuild society? Do they begin a guerrilla campaign against the cultists?
A defeat here, and this should certainly go into the Loss column, doesn't mean the end.